A steam turbine is also referred to as a turbomachine. As a rule, inter alia steam and gas turbines, water turbines and also centrifugal compressors are understood by the term turbomachine. These machines predominantly serve the purpose of extracting energy from a flow medium in order to drive another machine therewith, such as a generator.
Steam turbines as a rule are split into high-pressure, intermediate-pressure and low-pressure turbine sections. A turbine section, which on the inlet side is exposed to admission of superheated steam which may have temperatures of up to 620° C. and a pressure of up to 300 bar, is understood by a high-pressure turbine section in this application. The aforesaid temperature and pressure specifications are only indicative values. Turbine sections which are designed for higher temperatures and for higher pressures can also be referred to as high-pressure turbine sections. An intermediate-pressure turbine section is customarily exposed to admission of superheated steam which has a temperature of approximately 620° C. and a pressure of approximately 40 to 70 bar. A low-pressure turbine section is customarily exposed to admission of steam which issues from the intermediate-pressure turbine section. This steam which issues from the low-pressure turbine section is finally collected in a condenser and reconverted into water.
As a rule, the steam which issues from the high-pressure turbine section is heated in a reheater and flows into the intermediate-pressure turbine section.
The division into high-pressure, intermediate-pressure and low-pressure turbine sections is not consistently applied among experts. Thus, the steam parameters such as temperature and pressure cannot be applied as single differentiating criteria between a high-pressure, intermediate-pressure and low-pressure turbine section. An essential feature for identifying an intermediate-pressure turbine section is that this is exposed to admission of steam which comes from the high-pressure turbine section and is heated in a reheater to a higher temperature.
In a steam power plant, the thermal energy of the steam is predominantly converted into mechanical energy in order to drive a turbogenerator, for example. However, it is also known that steam turbines do not deliver only mechanical energy but also provide steam for specific purposes. For example, steam can be extracted from the steam turbine in order to provide process steam for a chemical process or to provide heating steam for district heating.
Such steam as a rule must be provided at a defined pressure. It is known to design a steam turbine in a steam plant in such a way that downstream of a specific stage a certain portion of steam is extracted at an extraction point which is provided for other purposes. The remaining steam is furthermore used for converting thermal energy into mechanical energy. However, it can happen that during operation of the steam turbine at partial load or with increased steam extraction, the pressure of the steam at the extraction point drops. Additional measures would then have to be adopted in order to achieve the required steam pressure.
The extraction of steam from the steam turbine for downstream processes may gain in importance within the scope of a subsequent CO2 separation. For this, a steam requirement which lies between 40% and 60% of the steam mass flow at the outlet of the intermediate-pressure turbine section at a pressure within the range of between 3.5 and 5.5 bar is expected.
Steam power plants with double-flow, intermediate-pressure turbine sections (see FIG. 1) are known, wherein the intermediate-pressure turbine section has a first flow and a second flow and the first and second flows in each case have a steam-extraction line which are formed in such a way that an external steam consumer is supplied with steam. The steam which flows from the first flow and from the second flow flows into the space between inner and outer casings which is connected to the crossover line and to the extraction connection, as a result of which a symmetrical construction of the steam turbine with regard to the waste steam system is realized. Furthermore, the first flow has a first turbine discharge line and the second flow has a second turbine discharge line. The first turbine discharge line and the second turbine discharge line lead into a crossover line which fluidically connects the intermediate-pressure turbine section to the low-pressure turbine section. A throttle valve, which sets the pressure in the crossover line and therefore also sets the pressure in the steam-extraction line, is arranged in this crossover line. The steam-extraction line frequently reaches between 2 and 6 bar after the last stage in the second flow. Since the extraction volume which is required for extracting the CO2 from the solvent is almost proportional to the power output of the power plant, the throttle valve in the second turbine discharge line is almost completely closed within a wide power output range. The steam mass flow which is throttled via this throttle valve and the fluidic losses are therefore significantly reduced. The advantage is that such plants can be built without CO2 separation. As a result of a subsequent extension with CO2 separation, additional losses as a result of throttling barely arise.
It is disadvantageous in this case, however, that during extraction the total steam which flows into the low-pressure turbine section is throttled. On account of these throttling losses, the mechanical power output of the steam turbine is significantly reduced.
It would be desirable to provide suitable steam without greater pressure loss which leads to a lower mechanical power output.